The present invention relates to an electric motor system that may be used to adjust the position of a load in relation to a plane, and it is applicable for positioning a load associated with a platform or stage of a positioning device such as an XY table.
Various kinds of motors are used in industry to move loads and to control their positions. The linear electric motor, i.e. a motor that produces thrust in a straight line by electromagnetic interaction, is one such kind of motor.
For example, U.S. Pat. No. 5,808,381 for a xe2x80x9cLinear Motorxe2x80x9d discloses several configurations of linear motors using permanent magnets in conjunction with coils in air gaps disposed within an area having a magnetic flux. U.S. Pat. No. 5,087,844, also for a xe2x80x9cLinear Motorxe2x80x9d, discloses a moving coil linear motor with a pair of permanent magnet assemblies on both sides of a central yoke opposing each other via a gap therebetween. Multi-phase, interlaced coil assemblies are disposed in the air gaps such that the force produced in one permanent magnet assembly acts in the same direction as the other. The magnets are shaped in such a way that the variation of magnetic flux density in the direction of motion of the moving part more closely approximates the ideal sinusoidal form than in conventional linear motors.
In the field of semiconductor assembly machines, the past decade has seen a shift from rotary motors and ball screw-based translation stages to linear motor based ones. This is so especially in the area of high speed, high precision XY tables used to move a load in two separate axes on a plane. XY tables comprise separate moving tables or platforms, each capable of moving in a single axis. A combination of movement in both axes allows an object to be positioned in any position on a plane. For instance, XY tables can be used as positioning devices in semiconductor assembling equipment, machine tools and measurement equipment.
In the conventional design of a linear motor driven XY table, each of the two axes on a plane is actuated by a linear motor in closed loop servo control, while feedback is provided by digital encoders, one for each axis. A variety of designs of linear motor may be used with such design of XY table.
Moving coil motors without iron cores are by far the most commonly-used motors for XY tables in the backend semiconductor industry. Whether single phase or multi-phase, they entail the use of a cantilevered coil bracket. The cantilever contributes substantially to the moment of inertia of a moving mass (including the load) about the axes of motion and also about the axis perpendicular to the axis of motion. This is undesirable from the point of view of vibrations in the yaw and pitch modes. Hence one of the goals of the motor designer is to keep the coil assembly as compact as possible and thereby to minimize the cantilever length. Thus, in a multi-phase motor, distributed coil windings are preferred over concentrated windings. Another advantage of distributed coil windings is better dissipation of the heat generated. FIG. 6c is an example of concentrated coil windings while FIGS. 6a and 6b are examples of distributed coil windings.
An example of an XY table comprising one linear motor for each axis is disclosed in U.S. Pat. No. 6,244,192 for an xe2x80x9cXY Tablexe2x80x9d. The improvement of this design over the prior art is mentioned as the use of linear motors instead of ball feed screws connected to driver motors when positioning a load along the X and Y axes. Amongst other things, ball feed screws are less accurate due to errors in screw connection. Further, high-speed acceleration and deceleration is not possible.
Nevertheless, one problem relating to control of high bandwidth servo controlled motors such as that used in the aforesaid patent is the effect of the offset current in the driver circuit which may vary with time due to changes in temperature and other aging effects in the circuit. This offset current manifests as an undesired motor force equal to the product of the motor force constant and the offset current. The force constant of a motor can be defined as the force produced per unit current. For a multi-phase motor, the force constant is normally based on the peak value of the sinusoidal current in each of its phases. A linear motor having a high force constant is sensitive even to small amounts of driver offset and spurious noise.
It is often difficult to meet the contradictory requirements of high precision and high speed while using just one motor, which has only one force constant. When the force constant of the motor is higher, the larger will be the undesirable force from the effects from interferences such as driver offset and spurious noise. Thus, it becomes difficult to control such a motor, especially in maintaining a steady stationary position at the end of its stroke. However, from the point of view of faster operation with a compact motor without need for a very high current from the driving circuit, the force constant of the motor needs to be adequately high. It follows that the disadvantage of controlling each axis with a single motor is that speed has to be traded off against precision.
It would be advantageous to be able to deliver a high force employing a high force constant motor to a table holding a load while moving it over a greater distance and also to deliver a lower force when positioning with more precision over a smaller distance is desired.
It is an object of the invention to seek to provide an improved motor system for positioning a load with greater speed and precision as compared to the prior art as discussed above.
According to a first aspect of the invention there is provided a motor system for positioning a load, comprising support means for supporting the load and a first motor and a second motor, the first motor and second motor cooperating to drive the load, whereby the first motor is adapted to provide higher thrust than the second motor and the second motor is adapted to deliver higher positioning accuracy than the first motor during a positioning sequence to position the load.
According to a second aspect of the invention there is provided a method for positioning a load, comprising the use of a first motor cooperating with a second motor to drive the load, wherein the first motor provides higher thrust to the load, and the second motor delivers higher positioning accuracy to the load.
Using the invention, it is possible to provide two linear motors working in tandem that may move a load on an XY table, to provide fast and accurate positioning of the load. The overall size of a cantilever of a moving coil motor may also be reduced in overall size, rendering the system compact and with a lower moment of inertia about the centre of gravity perpendicular to the axis of motion. Using the invention it may also be possible to facilitate the segregation and dispersion of heat loss over a wider area, thus leading to a lower temperature of operation.